Pulsed laser deposition is a known technique for the deposition of thin films of a material on a substrate. This process is widely published and is described in U.S. Pat. No. 4,701,592 entitled, "Laser Assisted Deposition and Annealing". The advantages of pulsed laser deposition for the deposition of multicomponent materials from a single composite target while maintaining its stoichiometry is also well known. This advantage of congruent evaporation has been discussed in the literature (see e.g., G. K. Hubler, MRS Bulletin, p. 26-27, February 1992 and J. T. Cheung and H. Sankur, CRC Critical Reviews in Solid State and Materials Sciences 15, p. 63-109). Other advantages include the ability to incorporate reactive species into the growing film by the presence of a suitable ambient gas in the deposition chamber (see e.g., G. K. Hubler, MRS Bulletin, p. 26-27, February 1992 and J. T. Cheung and H. Sankur, CRC Critical Reviews in Solid State and Materials Sciences 15, p. 63-109) and precise control of the deposition rate by controlling the fluence and repetition rate of the ablating pulses (see e.g., J. Cheung and J. Horwitz, MRS Bulletin, p. 30-36, February 1992). A desirable element of uv laser ablated film growth is the production of high energy plume species leading to the capability of growing epitaxial films on a low temperature substrate (see e.g., G. K. Hubler, MRS Bulletin, p. 26-27, February 1992; J. Cheung and J. Horwitz, MRS Bulletin, p. 30-36, February 1992; and S-G. Lee, D-S. Hwang, Y. K. Park and J-C. Park, Appl. Phys. Lett. 65 (6), p. 764-766, 1994).
The principal barrier to the use of pulsed laser deposition in the growth of defect free films is the inherent problem of "splashing" which causes the deposition of particulates in the growing film. This problem has been widely recognized and the detrimental effects of splashing on the quality of the deposited film has been pointed out (see J. T. Cheung and H. Sankur, CRC Critical Reviews in Solid State and Materials Sciences 15, p. 63-109). These particulates are generally undesirable for multilayer structures and fine line patterning for electronic applications (see S-G. Lee, D-S. Hwang, Y. K. Park and J-C. Park, Appl. Phys. Lett. 65 (6), p. 764-766, 1994, and A. Kuhle, J. L. Skov, S. Hjorth, I. Rasmussen and J. B. Hansen, Appl. Phys. Lett. 64 (23), p. 3178-3180, 1994). The optical losses produced by these particulates is of specific concern in optical films for waveguide lasers and optical couplers where even the presence of submicron particulates leads to an unacceptable scattering mechanism. As will be apparent in the following, the present invention is directed to a novel dual-laser process that substantially eliminates particulates in the deposited film.
In the literature, the ejection of microscopic particulates from the target during pulsed laser deposition stems primarily from two sources. These are the (&gt;1 .mu.m) particles that result from microcracks, pits and loosely attached particles on the target surface and the smaller submicron particles resulting from the superheating of a subsurface layer leading to explosive evaporation (see J. T. Cheung and H. Sankur, CRC Critical Reviews in Solid State and Materials Sciences 15, p. 63-109). Various attempts have been made to eliminate such particulates. A gravity dependent mechanical filtering approach has been reported (see J. T. Cheung, Appl. Phys. Lett. 43, 255, 1983) but is applicable only to the larger particles. However, the use of evaporation from a molten surface to prevent the generation of particulates is restricted only to single component targets. It has also been reported that reducing the power of the ablating laser below 150 mJ/cm.sup.2 minimizes the splashing (see e.g., S. D. Harkness and R. K. Singh, J. Appl. Phys. 75 (1), p. 669-671, 1994). However, the resultant decrease in plume species energy necessitates the use of higher substrate temperatures for epitaxial film growth, thus negating one of the most important advantages of pulsed laser deposition for in situ epitaxial film growth. An experiment using an excimer laser for heating the plume produced by a 1.06 .mu.m Nd:YAG laser about 2 mm away from the target has been reported to have shown some reduction in particulate deposition but not substantially complete removal (see e.g., G. Koren, R. J. Baseman, A. Gupta, M. I. Lutwyche and R. B. Laibowitz, Appl. Phys. Lett. 56 (21) p. 2144-2146, 1990). However, applicants believe the quality of the deposited films using this approach is inferior to the quality achievable by single excimer laser deposition. More recently, the use of a relatively high pressure inert ambient during single-laser deposition has been reported as indicating the possibility of removing the larger particulates (see A. Kuhle, J. L. Skov, S. Hjorth, I. Rasmussen and J. Bindslev Hansen, Appl. Phys. Lett. 64 (23) p. 3178-3180, 1994). Micron size particles still persist. Moreover, while eliminating some of the particulates the presence of the high pressure ambient drastically reduces the kinetic energy of species in the plume; again requiring elevated substrate temperatures to ensure film quality. Another approach to solving the particulate problem is described in U.S. Pat. No. 5,049,405, which discloses single excimer laser-ablated deposition where the use of simultaneous rotation and translation of the target is designed to ensure that the excimer laser ablates a fresh spot every shot. This process is still limited by the particulates existing in single laser deposition from even a smooth target.